Various types of web threading apparatus for association with rotary printing machines have been proposed. One such apparatus is shown in German Patent No. 24 02 768 to which U.S. Pat. No. 3,995,553, assigned to the assignee of this application corresponds. This arrangement discloses parallel guide tracks for a roller chain transporting the web through the machine. The roller chain is not as long as the length of the machine, but is sectioned, with drive wheels or sprockets being located at suitable distances, preferable equal, throughout the machine, to be selectively energized as the roller chain carrying a web therewith approaches a sprocket, the sprocket then engaging the roller chain to transport it in the threading path to the next sprocket. Although the probability is small, the situation may arise that the sprocket teeth of a stationary drive wheel are in the worst possible position with respect to the approaching roller chain, resulting in jamming of the first roller, which is pushed towards the sprocket wheel with a predetermined drive speed. The chain can jam, i.e. the flank of the moving chain may wedge with respect to the opposite guide rail portion. Malfunction of the threading apparatus results, and the consequent interruption is difficult to clear.